1. Field of the Invention
The invention relates to an electrolytic capacitor including an electrically conductive solid layer formed in a capacitor element having an anode foil and a cathode foil.
2. Description of the Related Art
As digitization of electronic equipment proceeds, it has become required that capacitors used therefor have a smaller size, a larger capacitance and a lower Equivalent Series Resistance (hereinafter, referred to as “ESR”) in a high-frequency range.
In order to reduce the ESR in a high frequency range, a solid electrolytic capacitor using, an electrically conductive material such as an electrically conductive polymer containing polypyrrole, polythiophene, and derivatives thereof as an electrolyte with higher electric conductivity than that of a conventional driving electrolytic solution has been known (see, for example, Japanese Examined Patent Publication No. 3040113).
Further, in terms of an increase in the capacitance, a solid electrolytic capacitor having an electrically conductive solid layer containing the above-described electrically conductive solid polymer which is formed in a wound capacitor element or a laminated capacitor element, has been commercialized. The wound capacitor element has a configuration in which an anode foil and a cathode foil are wound with a separator interposed therebetween and the laminated capacitor element has a configuration in which a plurality of anode foils and cathode foils are laminated.
However, with respect to the solid electrolytic capacitor described above, since an electrically conductive solid polymer having low repairability of a dielectric layer as an electrolyte is employed, the leak current tends to increase. Namely, in a case of an electrolytic capacitor filled with an electrolytic solution, since the electrolytic solution is brought into contact with a damaged portion of a dielectric layer, the damaged portion can be repaired by oxidation reaction with oxygen generated from a supporting salt of an ionic compound in the electrolytic solution when a rated voltage is applied. On the other hand, in a case of a solid electrolytic capacitor in which an electrically conductive solid layer is formed, since essentially no ionic migration occurs, the repair action as described above can scarcely be expected.
From the above-described point of view, a solid electrolytic capacitor, in which a capacitor element includes both of an electrically conductive solid layer containing an electrically conductive solid polymer and an electrolytic solution containing a supporting salt, has been proposed (see, for example, Japanese Unexamined Patent Publication No. 2006-100774).
The above-described solid electrolytic capacitor having both of a electrically conductive solid layer containing an electrically conductive polymer and an electrolytic solution containing a supporting salt in the capacitor element is, for example, produced in the following manner. First, an anode foil having a dielectric layer thereon and a cathode foil are wound with a separator interposed therebetween to form a capacitor element. Next, this capacitor element is impregnated with a polymerization solution containing a polymerizable monomer such as pyrrole, thiophene, and derivatives thereof, an oxidizing agent such as ammonium persulfate, and sodium persulfate, and a doping agent such as naphthalenesulfonic acid. Then, the above-mentioned polymerizable monomer is oxidatively polymerized in the inside of the capacitor element to form the electrically conductive solid layer.
An electrolytic capacitor has been required not to cause short circuit or ignition due to short circuit even if an overvoltage exceeding the rated voltage is applied. For this purpose, a structure for preventing emission of a gas generated inside an electrolytic capacitor by sealing an outer casing with a sealing body is employed for conventional electrolytic capacitors. However, if the short circuit of a capacitor element itself is prevented at a time of overvoltage application, higher safety can be assured.